The present invention concerns a selector device, and, more particularly, to an automatic selector device of a motor vehicle epicyclic change-speed gearbox, in which a frictional connection, i.e. a clutch or brake, participating in the selection of a gear can be engaged by a pressure medium selector setting element of the axial piston type but can be disengaged by a spring device to ensure a clearance between the friction surfaces in the rest condition. A working pressure conduit starting from the selector setting element, a reservoir pressure conduit starting from a pressure reservoir, a working pressure intermediate conduit starting from a 3/2-way selector valve and connected to a controller pressure conduit and a pressure-relieved return are connected to a disengagement control valve in which two positions can be selected. The intermediate conduit is connected to the controller pressure conduit in the gear position of the selector valve associated with the selection of the gear and is connected to the return in the deselection position of the selector valve corresponding to the deselection of the gear, and the working pressure conduit is connected, on one hand, to the intermediate conduit in one position of the disengagement control valve and, on the other hand, is shut off from the intermediate conduit in the other position of the disengagement control valve.
In a known selector device of the type shown in German Offenlegungsschrift No. 23 21 352, the selector setting element of a plate clutch participating in the selection of the third gear is connected to the intermediate conduit by a bypass conduit starting from the working pressure conduit via a non-return valve opening in the direction of the selector setting element while bypassing the disengagement control valve. The reservoir pressure conduit is itself connected to the bypass conduit via a throttle so that the pressure reservoir is charged from the intermediate conduit via the bypass conduit when the clutch is engaged. A control pressure conduit branches off from the reservoir pressure conduit and is connected, together with a second control pressure conduit carrying a control pressure which is a function of the vehicle speed, to the disengagement control valve.
Although the intermediate conduit of this known selector device is connected to the return of the selector valve if the associated control valve is switched into its position for the second gear, the selector setting element of the clutch is nevertheless cut off from the return by the non-return valve and by the disengagement control valve until a gear brake participating in the selection of the second gear is more or less engaged, with the pressure reservoir emptying via the bypass conduit to the return of the selector valve and the selector setting element emptying via the disengagement control valve to the return of the selector valve. In this configuration, a transfer of pressure medium from the selector setting element into the pressure reservoir is neither intended nor possible.
In order to reduce the fuel consumption of city buses with an epicyclic change-speed gearbox, an attempt is made to switch the gearbox to "neutral" automatically each time the vehicle comes to rest so that the engine is no longer loaded when idling and thus the fuel injection quantity is reduced. Because the vehicle spends a large proportion of its time at rest in city center stop-and-go traffic due to traffic and bus stops, and also the driver cannot be expected to actuate the neutral button at every stop, the potential for achieving savings is substantial.
Although there are no problems with an automatic selection of neutral on a level roadway, the danger of rolling back occurs during stops on gradients if, in order to move off, the driver removes his foot from the brake pedal and presses the accelerator. This is because a frictional connection has to be engaged in the gearbox in order to reestablish the force path. It is then important to keep the filling procedure of the relevant selector setting element as short as possible.
A frictional connection, which has to transmit an excessive torque in driving operation, sometimes has to be released in "neutral" in the case of an epicyclic change-speed gearbox. A clutch for the introduction of the drive torque in the forward gears, which could theoretically also be used for the automatic selection of the neutral position, must not be unloaded in those cases in which this clutch forms a structural unit with a clutch for the introduction of the drive torque in reverse gear. Because of the centrifugal force of its actuating medium, this clutch would engage. Since the frictional connection transmitting an excess torque in one gear is relieved of load in other gears, it is necessary to ensure an adequate clearance between the friction surfaces in order to avoid high drag losses when the vehicle is at rest.
In the unloaded condition, the return of the actuating piston of the frictional connection is ensured by return springs. When the frictional connection is selected, the piston space must first be refilled before all the plates come into contact with one another and torque can be transmitted. The pressure supplied during his filling procedure must not be too large so that, when it is finished and, because the pressure drop is no longer present, the pressure in the selector setting element climbs to the same level as the pressure supplied no excessively large torque is transmitted by the frictional connection. As a result of the relatively low pressure supplied, however, the filling procedure last too long and the vehicle can roll back as previously discussed.
An object of the invention is to reduce the selection period of the frictional connection when selecting from neutral to such an extent that, when starting on a hill, the vehicle does not roll back to any noticeable extent and, at the same time, sufficient plate clearance is ensured in the frictional connection in normal driving operation in other gears.
The foregoing objects have been achieved in an advantageous manner in accordance with the present invention by providing that in a neutral selection condition cancelling the torque transmission in the change-speed gearbox, the selector valve is in its gear position and the disengagement control valve is in its disengagement position shutting off the working pressure conduit from the intermediate conduit. The disengagement control valve is still connected to a pressure-relieved return and, via the working pressure conduit, is exclusively connected to the selector setting element and, via the reservoir pressure conduit, is exclusively connected to the pressure reservoir. The reservoir pressure conduit is, on one hand, connected to the working pressure conduit in the disengagement position of the disengagement control valve but, on the other hand, is connected to the return in the gear position of the disengagement control valve, and the volume of the pressure reservoir is matched to the volume, occurring in the engaged position, of the working pressure chamber of the selector setting element such that, when the working pressure conduit is connected to the reservoir pressure conduit via the disengagement control valve, the selector setting element can only be disengaged to the extent that the frictional connection is just unloaded and does not transmit torque.
In the selector device according to the present invention, an additional pressure reservoir is provided which can be connected via the disengagement control valve.